It is commonly accepted that humans, and primates in general, are a relatively unmatched species when it comes to the brain. For example, a primate's brain has a larger pre-frontal cortex — the area used for making decisions and where personality is housed, something which is thought to be specific to primates and humans.
Another characteristic of the primate brain is the loss of neurons through aging in the prefrontal cortex. That is a fact which has been known for a while. What was unknown, however, was whether such neuron loss was special to the primate prefrontal cortex alone or whether it could be seen in other species. Researchers in the Gallagher Lab of the Department of Psychological and Brain Sciences at Hopkins sought to start unraveling this problem by comparing the number of stained principal neurons and interneurons in the dorsal prefrontal cortex of both younger and older rats.
The prefrontal cortex in a rat, unlike that of a primate's, can be divided into two regions: the ventral and the dorsal region. The ventral region is associated with the hippocampus, an area of the brain that is in charge of memory and spatial navigation and other related structures. The dorsal prefrontal cortex in a rat has connections to and from parts of the brain which concern motor, somatosensory, visual and retrosplenial functions.
In order to determine if neuron reduction occurred in the older rates, researchers stained some neurons in rats' prefrontal cortex and waited to see if they were reduced as the rats grew older. Results show that there was neuronal loss in rats, but only in the dorsal region and only the principal and immunoreactive cells were lost. No changes in neuron number were observed in the ventral prefrontal cortex at all.
Rats were also tested to see if reduction of neurons in the dorsal region of the prefrontal cortex would correspond with loss of function by comparing performance in a water maze to loss of neurons. The lab found that neuronal loss occurred in rats which performed both well and badly in the maze, leading them to conclude that neural reduction at least did not affect spatial navigation. Therefore, there may or may not be some sort of mechanism used to compensate for the loss of neurons and account for the similarity in performance in the maze.
There have been other behavioral patterns in older rats which have been observed in the past, but they may or may not be due to the loss of prefrontal neurons. One such pattern is bad performance during attentional set-shifting. Further testing is needed to determine whether this, and other such behavioral patterns, are due to neuronal loss.
However, because the dorsal prefrontal cortex is connected to so many other parts of the brain, there have been connections found which show that the loss of neurons is associated with overall slower cognitive functioning. This may mean that losing neurons in the dorsal prefrontal cortex affects many other areas of functioning, including information processing and general cognition.
The results from this study contradict the results from a previous study, where neuronal loss in the ventral prefrontal cortex was observed, but neuronal loss in the dorsal prefrontal cortex was not. It has been suggested that either a difference in how the cortex was divided into ventral and dorsal areas, or that genetic drift due to use of two different types of rats in each respective experiment, accounts for this disparity.
Whatever the reasons, both of these result open the possibility that brains of primates may have more in common with other species than previously thought. Further testing on other aspects of the brain will be required in order to determine whether similarities extend beyond neuronal loss, but the results from this experience show that humans and primates might not be as special as they were originally thought to be.
